This invention relates to multilamp photoflash devices having circuit means for sequentially igniting the flashlamps and, more particularly, to improved means for permitting reliable flashing of an array of photoflash lamps, particularly arrays operated by comparatively long duration, low voltage firing pulses.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art, particularly in the past few years. Series and parallel-connected lamp arrays have been shown which are sequentially fired by mechanical switching means, simple electrical circuits, switching circuits using the randomly varied resistance characteristics of the lamps, arc gap arrangements, complex digital electronic switchin circuits, light-sensitive switching means and heat-sensitive switching devices which involve melting, fusing or chemical reaction in response to the radiant energy output of an adjacently located flashlamp. The present invention is concerned with an improved radiant-energy-activated switching means useful in a relatively inexpensive photoflash unit of the disposable type. In particular, the present switching means is particularly advantageous in photoflash arrays emloying lamps adapted to be ignited sequentially by successively applied firing pulses from a low voltage source.
A currently marketed eight-lamp photoflash unit employing radiation switches is described in U.S. Pat. Nos. 3,894,226 and 4,017,728 and referred to as a flip flash. A ten lamp version is described in U.S. Pat. Nos. 4,156,269 and 4,164,007. The unit comprises a planar array of high voltage flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The circuit board comprises an insulating sheet of plastic having a pattern of conductive circuit traces, including terminal contacts, on one side. The flashlamp leads are electrically connected to the circuit traces, such as by means of eyelets, and the circuitry on the board includes a plurality of solid state switches that chemically change (convert) from a high to low resistance so as to become electrically conducting after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. The purpose of the switches is to promote lamp sequencing and one-at-a-time flashing.
One type of solid state switch which operates in this manner is described in U.S. Pat. No. 3,458,270 of Ganser et al, in which the use of silver oxide in a polyvinyl binder is taught as a normally open radiant energy switch. Upon radiant heating, the silver oxide decomposes to give a metallic silver residue which is electrically conductive. Silver carbonate has also been used in lieu of or together with silver oxide. For example, U.S. Pat. No. 3,990,833, Holub et al, describes a mass of a composition comprising silver oxide, a carbon-containing silver salt and a humidity resistant organic polymer binder, the switch mass being deposited on a circuit board so as to interconnect a pair of spaced apart electrical terminals formed by the printed circuitry thereof. A similar type radiation switch exhibiting an even greater humidity resistance at above normal ambient temperatures is described and claimed in U.S. Pat. No. 3,990,832, Smialek et al, which describes the use of a particular stabilizer additive, such as an organic acid, to preclude or reduce the tendancy of the silver source in the switch material from premature conversion to a low electrical resistance when exposed to ambient humidity conditions. U.S. Pat. No. 3,951,582, Holub et al, describes a similar type switch with a colored coating, and U.S. Pat. No. 4,087,233, Shaffer, describes a switch composition comprising silver carbonate, a binder, and an oxidizer such as barium chromate, which is particularly resistant to high relative humidity and above normal ambient temperatures. U.S. Pat. No. 3,969,065, Smialek, describes a solid state switch comprising a mixture of solid copper salt with a humidity resistant organic polymer binder and a finely divided metal reducing agent, and a U.S. Pat. No. 3,969,066, Smialek et al, describes a switch comprising a mixture of finely divided cupric oxide with a humidity resistant organic polymer binder.
The use of a glass bead filler in a solid state switch is described in U.S. Pat. No. 4,080,155 of Sterling for preventing the switch material from being burned off or blown off the circuit board. An improved switch composition which avoids these problems is described in copending application Ser. No. 021,398, filed Mar. 19, 1979, and assigned to the present assignee. The improved burn-off prevention and reduced heat absorption are attained by replacing part of the silver carbonate and/or silver oxide in the switch composition with a proportion of electrically nonconductive inert particulate solids which comprise as much as 25-65% by weight of the total dried composition.
All of the aforementioned switch compositions have been described with respect to use in photoflash arrays employing high voltage type lamps adapted to be ignited sequentially by successively applied high voltage firing pulses from a source such as a camera-shutter-actuated piezoelectric element. Accordingly, none of these prior patents or applications mention a specific switch closure interval, i.e., the time of conversion from a high electrical resistance to a low electrical resistance upon exposure to radiation emitted from an adjacent flashlamp. Consideration of such switch closure, or conversion, time has not been necessary in a high voltage piezo-fired array since the electrical firing pulse duration is less than 10 microseconds, whereas the normally open radiation switch is not activated until 5-10 milliseconds; i.e., the conversion time is 5 to 10 milliseconds. If it is desired to use such normally open radiation switches in a low voltage photoflash array intended for operation with a typical camera actuated, battery powered pulse source, the reliability of proper lamp sequencing can be adversely affected. In a low voltage array, the electrical pulse duration can extend to a period longer than the conversion time of the normally open radiation switch. If the pulse duration is long enough, a second lamp will inadvertently flash, thereby causing the loss of that lamp in the intended useful sequence of lamp operation. Accordingly, it is desirable to have a normally open radiation switch that will activate (be converted) after the camera pulse has ended. By way of specific example, consider a low voltage camera having a pulse duration which varies from 4 to 10 milliseconds. In such an application, a switch conversion time of greater than about 12 milliseconds would be required. For these purposes, switch conversion time is defined as the elapsed time between the start of the firing pulse, and thus the high electrical resistance (open circuit) state of the switch mass, and the time at which the switch resistance reaches a predetermined low resistance state, which functions as a closed circuit in the operating application.